The invention relates to the field of gamma-ray detectors, and especially gamma-ray detectors using the method of coincidence detection of positron emission for nuclear medical imaging.
Coincidence detection methods for positron emission, also known as electronic collimation, is used in the field of nuclear medical imaging. In this method isotopes that emit positrons are injected into the body of the examined patient. Each of the emitted positrons annihilates with an electron to produce a pair of 511 Kev photons propagating along the same line but in opposite directions and out of the patient""s body. The 511 Kev photons are detected by a camera which has two separate detector heads, which determine the position where the photons interact with the detector heads and the energy of these interacting photons.
Only pairs of photons with both of their measured energies within the predetermined energy range 511+/xe2x88x92xcex94E Kev are suitable for composing the image. The positions of interaction of these photons in the plane of the two separate detector heads define a line, which passes through the location of the point of origin of the electron-positron emission in the patient""s body. By deriving these lines from the useful photons measured, an image can be constructed from the calculated positions of origin of the pairs.
Photons with measured energy of 511+/xe2x88x92xcex94E Kev are used for acquiring the image, where xcex94E is a predetermined energy range, which depends, among other factors, on the energy resolution of the camera and on the quality of the desired image. Photons with measured energy outside of the range of 511+/xe2x88x92xcex94E Kev are probably photons that lost part of their energy by a scattering process in the patient body known as Compton scattering. Compton scattering changes the path of photons from the original orientation in which they were emitted by the annihilation process. Accordingly, photons that have undergone Compton scattering are not suitable for image acquisition, and the image-processing unit of the camera thus ignores them.
Photons of the same pair are emitted simultaneously. Accordingly, the detection time of one photon of a pair should differ from the detection time of the second photon, only by a very small time interval xcex94t which depends, among other factors, on the time resolution of the system, and on the different time of flight of each photon to its corresponding detector head.
The rate of the measured events in the detector heads determines the average time xcex94T between two followings events. Two photons are considered as being related to the same pair when they are detected by the two detector heads of the camera within a time difference xcex94t, which satisfies the condition xcex94t less than xcex94T.
Accordingly, measuring the detection times of the photons by the detector heads implies that two photons (one in each head) are related to the same pair, when xcex94t less than xcex94T.
For more accurate imaging, the exact location of the electron-positron emission point on the above-derived lines, can be found from the calculations of the time of flight xcex94t1 and xcex94t2 of the photons, from their emission sites to the two detector heads.
In a typical coincidence method xcex94t is approximately 10 nanoseconds. This means that the time-resolving capability of the camera in measuring the interaction time of the photons with the detector heads should also be better than 10 nanoseconds.
In Anger cameras in which the photon detection is done by a combination of scintillator and photomultipliers, a time resolution of 10 nanoseconds is achieved on a regular basis.
In a pixellated solid-state detector array, the rise-time of the signal in the pixellated anodes depends on the weighting potential (commonly known as the xe2x80x9csmall pixel effectxe2x80x9d), as described in the article entitled xe2x80x9cSignals induced in semiconductor gamma-ray imaging detectorsxe2x80x9d by J. D. Eskin et al., published in Journal of Applied Physics, Vol. 85, pp. 647ff. (1999), hereby incorporated in its entirety by reference. The weighting potential depends mainly on the ratio between the dimensions of the anodes and the detector thickness. For example, in a typical pixellated solid-state detector, made of CdZnTe and designed for coincidence method use, the above-mentioned ratio is about 1/4 (2.5 mm anode size in a 10 mm detector thickness). In this situation the interaction time measured by the anodes depends strongly on the depth of interaction in the semiconductor. As a consequence, the anodes would measure two different events that occur simultaneously but at different depth of interaction, as if they occurred at different times. For the specific example of the detector mentioned above, the error in measuring the time of interaction by the anodes, can be as large as 1 microsecond.
An error of 1 microsecond in measuring the time of interaction is totally unacceptable for a coincidence measurement method, where the measurement should have a time resolution of 5 nanosecond.
An alternative technique for using the coincidence method with pixellated semiconductor detectors is to derive the time of interaction from the signal produced by the cathode of the detector head. The cathode is very big in comparison to the pixellated anodes and thus the small pixel effect in this case is negligible. The signal produced by the cathode is effectively instant and the time of interaction measured by this signal is thus independent of the depth of interaction.
However, the use of the detector head cathode for measuring the time of interactions introduces two major problems:
1. The large cathode suffers from a large dark current that produces a high noise level. The high level of noise in the cathode may cause a significant error in measuring the time of interaction.
2. Even though the time of coincidence is measured accurately using the cathode signals, there is still a problem of how to define the correct anodes to which the coincident signals belong. Anodes detecting two events that may occur up to 1 microsecond apart, may detect these events as occurring at inaccurate times and even in the reverse order to their true occurrence, because of the interaction depth delay.
There therefore exists a serious need for a device and method for deriving a low noise signal by which the interaction time of a photon with a pixellated semiconductor detector head can be accurately measured independently of the depth of interaction. At the same time, the device and method should be capable of determining to which one of the signals of the pixellated anodes of the semiconductor detector each coincidence signal is related.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.
The present invention seeks to provide a new gamma ray detector which has a sufficiently fast response time for any impacting photon that it is able to detect coincidence events accurately, and which is capable of identifying the correct pixellated anodes in the camera heads to which the coincidence events are related, and which also operates at a low dark current noise level.
According to one preferred embodiment of the present invention, this objective is achieved by means of a detector array having segmented cathodes, the area of each segment being large enough not to engender small pixel effects which would slow down the time of response of the cathode, and having pixellated anodes of the dimensions required to provide the desired resolution of the detector array. The segmented cathodes provide a fast coincidence signal, including the location address of the cathodes impacted by the coincident photons, and this coincidence signal is used to read the charge signals detected only on those pixellated anodes located behind the cathodes providing the coincidence signal. Each cathode area is sufficiently small, on the other hand, that there is very low likelihood of having two events occurring within the group of anodes behind those cathodes, within the time that it takes to readout the slowest responding impact on those anodes because of the small pixel effect.
According to further preferred embodiment of the present invention, instead of the segmented cathodes being used to provide the fast coincident trigger data, a segmented grid array on the anode side of the detector, and located between the individual anodes, may be used. Such a grid has been previously described in U.S. Pat. No. 6,034,373 to some of the inventors of the present application, hereby incorporated in its entirety by reference. The segmented grid covers a large effective area of the detector. The grid conductor itself is of smaller area but still does not engender small pixel effects, and furthermore, if used with an insulating base layer, is a non collecting electrode. Thus the grid provides a very low noise signal for the coincidence detection. Similar to the segmented cathodes, the segmented grid provides a fast coincidence signal, including the location address of the grid segment impacted by the coincident photons, and this coincidence signal is used to read the charge signals detected only on those pixellated anodes located within the grid segment providing the coincidence signal. Each area of a grid segment is sufficiently small, on the other hand, that there is very low likelihood of having two events occurring within the group of anodes, locating within the grid segment, in the time that it takes to readout the slowest responding impact on those anodes because of the small pixel effect. This geometry has a significant advantage in that the majority of the electrical connections can be made on one side of the detector array, thus simplifying design and construction.
There is further provided in accordance with yet another preferred embodiment of the present invention, a semiconductor coincidence detector device consisting of at least a first and a second detector crystal array, each having a first and second surface, an array of pixellated anodes formed on each of the first surface, an array of segmented cathodes formed on each of the second surface, essentially each of the pixellated anodes being connected to an anode electronic channel for generating a first electrical signal corresponding to the energy of a photon impinging in the semiconductor, and essentially each of the segmented cathodes being connected to a cathode electronic channel for generating an electrical coincidence trigger signal on detection of a photon impinging in the semiconductor, the first electrical signal being read only from anodes located opposite the cathode segment generating the trigger signal.
There is even further provided in accordance with yet another preferred embodiment of the present invention, a semiconductor coincidence detector device consisting of at least a first and a second detector crystal array, each having a first and second surface, an array of pixellated anodes formed on each of the first surface, an array of segmented grids formed on each of the first surface, essentially each of the pixellated anodes being connected to an anode electronic channel for generating a first electrical signal corresponding to the energy of a photon impinging in the semiconductor, and essentially each of the segmented grids being connected to a segmented grid electronic channel for generating an electrical coincidence trigger signal on detection of a photon impinging in the semiconductor, the first electrical signal being read only from anodes located within the area of the grid segments generating the trigger signal.